Chiller system with low capacity controller and method of operating same

ABSTRACT

A method ( 126 ) and a low capacity controller ( 28 ) enable a chiller system ( 20 ) to operate below a minimum allowable capacity. The chiller system ( 20 ) includes a chiller ( 40 ) and a chiller fluid loop ( 30 ), having a supply line ( 36 ) for conveying a chiller fluid ( 34 ) from the chiller ( 40 ) and a return line ( 38 ) for returning the chiller fluid ( 34 ) to the chiller ( 40 ). A capacity demand for the chiller ( 40 ) is determined. When the capacity demand is less than a minimum allowable capacity of the chiller ( 40 ), the chiller fluid ( 34 ) is routed from the return line ( 38 ) to a heat exchanger ( 96 ) of the low capacity controller ( 28 ) where the chiller fluid ( 34 ) is warmed and returned to the return line ( 38 ). The warmed chiller fluid ( 34 ) establishes a false capacity demand, detectable at the chiller ( 40 ), that is at least the minimum allowable capacity.

RELATED INVENTIONS

This is a divisional application of “Chiller System With Low CapacityController And Method Of Operating Same,” filed 13 Jul. 2004, Ser. No.10,890,585.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of chilled water systems.More specifically, the present invention relates to a method ofoperating a chilled water system.

BACKGROUND OF THE INVENTION

Chiller systems provide a temperature conditioned fluid, for use inconditioning the air within large buildings and other facilities. Thechilled fluid is typically pumped to a number of remote heat exchangersor system coils for cooling various rooms or areas within a building. Achiller system enables the centralization of the air conditioningrequirements for a large building or complex of buildings by using wateror a similar fluid as a safe and inexpensive temperature transportmedium.

In general, a chiller of the chiller system provides chilled water of aparticular temperature, via a first fluid loop, for cooling air in abuilding. Heat is extracted from the building air, transferred to thefluid in the first fluid loop, and is returned via the first fluid loopto the chiller. The returned fluid is again cooled to the desiredtemperature by transferring the heat of the fluid to the chiller'srefrigerant. After the refrigerant is compressed by a compressor, theheat in the refrigerant is transported to the condenser and heat istransferred to a second fluid conveyed in a second fluid loop. Thesecond fluid loop transports waste heat from the condenser of thechiller to a cooling tower which then transfers the waste heat from thesecond water loop to ambient air by direct contact between the ambientair and the second fluid of the second loop.

A multiple chiller system has two or more chillers connected by parallelor series piping to a common distribution system. Multiple chillersoffer operational flexibility, standby capacity, and less disruptivemaintenance. Through the use of multiple chillers, when the coolingdemand is low, only one chiller may need to operate, and the operatingchiller's capacity may be controlled to match the demand. If the coolingdemand is beyond a single chiller's maximum capacity, one or moreadditional chillers may need to be activated. As such, the operatingchillers are controlled so the system's total capacity (sum of thechillers' individual capacities) meets the cooling demand.

Chiller capacities normally are based on the maximum load anticipated.However, much of the time that a chiller is in use, it is operating atless than full load. Indeed, a chiller system in a building may operateover a wide range of demand conditions, with significant dominance oflow capacity demand. Low capacity demand can result from seasonalfluctuations, when cooling a small office or zone during “off-hours”,when there is low or no load available for the start up of the chillersystem, and so forth.

Low capacity demand typically causes a chiller to operate lessefficiently. That is, lower capacity demand results in a higher kilowattuse per ton. More critically however, low capacity demand can force achiller system to operate in unstable conditions. Under such conditions,compressor and evaporator capacities balance at ever lower suctionpressures and temperatures. Typically, chillers are outfitted withprotection mechanisms that cause them to shut down when the capacitydemand becomes very low, for example, less than approximately eighteenpercent of total chiller capacity. Left unchecked, the eventual resultis coil frosting and compressor flooding.

Accordingly, some chillers are unable to operate when the capacitydemand is very low. This is problematic for an operator who wishes tocool a small office or zone during “off-hours”, who needs to keep oneoffice or room open past normal closing time, who has low or no loadavailable for the start up of the chiller system, and so forth.

Attempts have been made to circumvent this problem by the inclusion of ahot gas bypass. A hot gas bypass can stabilize a chiller system bydiverting hot, high-pressure refrigerant vapor from the discharge linedirectly to the low-pressure side of the chiller. This technique keepsthe chiller compressor more fully loaded while the evaporator satisfiesthe part-load condition. In addition, the diverted vapor raises thesuction temperature, which prevents frost from forming.

Although hot gas bypass can provide frost control and match systemcapacity to load to allow the system to operate at safe balance pointsduring unsafe loads, hot gas bypass sometimes fails to safely stabilizethe chiller system. Moreover, hot gas bypass can undermine the reliableoperation of a chiller by introducing problems stemming frominsufficient oil return and refrigerant logging in the hot gas bypassline. In addition, hot gas bypass requires an additional refrigerantline thus increasing the initial cost of a chiller system, and alsoincreasing the likelihood of refrigerant leaks. Hot gas bypass furtherreduces operating efficiency because the bypassed vapor does no usefulcooling.

Accordingly, what is needed is a method and apparatus for operating achiller system below a minimum allowable capacity for the chiller systemwhen a capacity demand is very low.

SUMMARY OF THE INVENTION

Accordingly, it is an advantage of the present invention that a methodand low capacity controller are provided for operating a chiller system.

It is another advantage of the present invention that a method and lowcapacity controller are provided that enable a chiller system to beoperated below a minimum allowable capacity for the chiller system.

Yet another advantage of the present invention is that the low capacitycontroller can be readily and cost effectively incorporated into achiller system.

The above and other advantages of the present invention are carried outin one form by a method of operating a chiller system, the chillersystem including a chiller and a fluid loop, and the fluid loopincluding a supply line for conveying a fluid from the chiller and areturn line for returning the fluid to the chiller. The method calls fordetermining a capacity demand for the chiller. When the capacity demandis less than a minimum allowable capacity of the chiller, the fluid iswarmed in the return line to establish a false capacity demand,detectable at the chiller, that is at least the minimum allowablecapacity.

The above and other advantages of the present invention are carried outin another form by a low capacity controller in a chiller system. Thechiller system includes a chiller and a first fluid loop, the firstfluid loop including a supply line for conveying a chiller fluid fromthe chiller and a return line for returning the chiller fluid to thechiller, the chiller being operable above a minimum allowable capacityfor the chiller. The low capacity controller includes a heater, and asecondary loop interposed between the return line and the heater. Thelow capacity controller further includes means for enabling a transferof the chiller fluid through the heater via the secondary loop when acapacity demand is less than the minimum allowable capacity, the chillerfluid being warmed at the heater, and means for returning the chillerfluid from the heater to the return line via the secondary loop.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the Figures, wherein like reference numbers refer tosimilar items throughout the Figures, and:

FIG. 1 shows a block diagram of a chiller system in accordance with apreferred embodiment of the present invention;

FIG. 2 shows a flowchart illustrating an operational process of thepresent invention in accordance with the preferred embodiment;

FIG. 3 shows a block diagram of the chiller system operating in a lowcapacity mode in accordance with the preferred embodiment;

FIG. 4 shows a block diagram of the chiller system operating in achiller bypass/economizer mode;

FIG. 5 shows a block diagram of a low capacity controller for a chillersystem in accordance with an alternative embodiment of the presentinvention; and

FIG. 6 shows a block diagram of a low capacity controller for a chillersystem in accordance with another alternative embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a chiller system 20 in accordance with apreferred embodiment of the present invention. In general, chillersystem 20 includes a chiller fluid section 22, a refrigeration section24, a condenser fluid section 26, a low capacity controller 28, and asystem controller 29.

Chiller fluid section 22 includes a chiller fluid loop 30 and pumps 32.Pumps 32 are in fluid communication with chiller fluid loop 30 forforcing a chiller fluid, represented by arrow heads 34, to circulatewithin chiller fluid loop 30. Chiller fluid loop 30 includes a supplyline 36 and a return line 38. Supply line 36 conveys chiller fluid 34from chillers 40 of refrigeration section 24 to an air handler 42 forconditioning the air within a space served by air handler 42. Airhandler 42 uses chiller fluid 34 to transfer heat energy from the airbeing circulated from the space by means of a fan 44 and ductwork (notillustrated) to a heat exchange coil 46 of chiller fluid loop 30. Returnline 38 returns chiller fluid 34 to chillers 40, so that chiller fluid34 can be subsequently re-cooled and re-circulated through chiller fluidloop 30.

Chiller fluid section 22 further includes one or more pump controllersor variable flow devices 48 for controlling a flow rate of chiller fluid34 through chiller fluid loop 30 and a differential pressure sensor 50for controlling a bypass valve 52 to maintain a minimum flow of chillerfluid 34 in chiller fluid loop 30. Chiller fluid section 22 furtherincludes a flow meter 54, a supply temperature sensor 56, a returntemperature sensor 58, and a return mixed temperature sensor 60 forstaging chillers 40 on and off in response to a capacity demand(discussed below) and/or to control bypass valve 52 as known to thoseskilled in the art.

In a preferred embodiment, pumps 32 form a primary variable flowdistribution system. Such a primary variable flow distribution systemlowers initial costs, due to the elimination of the secondary pumps andassociated fittings, vibration isolation, starters, and so forth, anduses less energy than conventional primary-secondary variable flowsystems. However, the present invention may alternatively be adapted foruse in a primary-secondary variable flow distribution system. Inaddition, only two pumps 32 are shown for simplicity of illustration.However it should be understood that chiller system 20 may include anynumber of pumps 32.

Although only one air handler 42 is illustrated herein, it should bereadily apparent that chiller fluid loop 30 may distribute chiller fluid34 to various system coils or heat exchangers for cooling rooms or otherareas within a building.

Refrigeration section 24 includes one or more chillers 40. In thisexemplary embodiment, chiller system 20 includes two chillers 40.However, the present invention may be adapted for use in chiller systemsthat include any number of chillers 40. In addition, chillers 40 neednot be of the same capacity. As such, one of chillers 40 may have ahigher maximum cooling capacity than the other.

In an exemplary embodiment, chillers 40 are screw compressors utilizingR134A refrigerant. Screw compressors are positive-displacement machineswith nearly constant flow performance, and are usually available inunits from about 30 to 1250 tons. A slide valve of a screw compressor isused to adjust the refrigerant's flow rate for varying the chiller'scapacity or cooling effect. Although screw compressors are describedherein, it should be apparent to those skilled in the art that othertypes of chillers may be employed as an alternative to screwcompressors.

Generally, each of chillers 40 includes a compressor 62 that forces arefrigerant in series through a condenser 64, an expansion device 66(for example, a flow restrictor, orifice, capillary, expansion valve,and so forth), and an evaporator 68 via a refrigeration conduit 70.Evaporator 68 conditions chiller fluid 34 to a predeterminedtemperature, for example, 45° F., so that chiller fluid 34 can be reusedand conveyed in chiller fluid loop 30 to air handler 42. The energyextracted from chiller fluid 34 by evaporator 68 is transported byrefrigeration conduit 70 to compressor 62 which lowers the condensationpoint of the refrigerant so that the refrigerant can be condensed bycondenser 64.

Condenser fluid section 26 includes a cooling tower 72, pumps 74, and acondenser fluid loop 76 conveying a condenser fluid, generallyrepresented by arrow heads 78. Second fluid loop 76 is interposedbetween cooling tower 72 and condenser 64 of each of chillers 40. Whenthe refrigerant is condensed by condenser 64, the energy, in the form ofheat is transferred to condenser fluid 78 in condenser fluid loop 76.Pumps 74 force the circulation of condenser fluid 78 through coolingtower 72 where the heat of condenser fluid 78 is transferred to ambientair.

Condenser fluid section 26 may further include one or more flow valves80 and one or more check valves 82 associated with pumps 74 thatregulate a flow rate and flow direction of condenser fluid 78 incondenser fluid loop 76. Condenser fluid section 26 may further includea differential pressure sensor 84 for controlling a bypass valve 86 tomaintain a minimum flow of condenser fluid 78 in condenser fluid loop76. In addition, a condenser fluid temperature sensor 88 and a condenserfluid flow meter 90 may be provided for monitoring the temperature andflow rate of condenser fluid 78 in condenser fluid loop 76.

As known to those skilled in the art, instead of operating energyintensive chillers during cool, dry outdoor conditions, the coolingeffect of outside air may be utilized to provide direct cooling to afacility or process using cooling tower 72. This situation is sometimesknown as “free cooling,” and the chillers are deactivated, i.e., theoperation of chillers 44 is bypassed. Thus, a three-way valve 92 may beincluded in condenser fluid section 26 for selectively routing condenserfluid 78 through cooling tower 72, or for bypassing cooling tower 72. Inaddition, when chillers 44 are deactivated, condenser inlet valves 94may be provided that close partially or entirely to limit or prevent aflow of condenser fluid 78 into condensers 64 when utilizing “freecooling.”

In accordance with a preferred embodiment of the present invention,chiller system 20 includes low capacity controller 28. Low capacitycontroller 28 advantageously enables chillers 44 to operate below aminimum allowable capacity for chillers 44. In general, the minimumallowable capacity may be approximately eighteen percent of totalchiller capacity. Conventional chiller systems are outfitted withprotection mechanisms that cause them to shut down when the capacitydemand becomes very low, for example, less than approximately eighteenpercent of total chiller capacity. Left unchecked, the eventual resultis coil frosting and compressor flooding. Accordingly, conventionalchiller systems cannot operate at very low capacity demands.

Low capacity controller 28 of the present invention enables chillers 44to operate at a capacity demand much lower than eighteen percent oftotal capacity, for example, as low as approximately three percent oftotal capacity or lower. It will become readily apparent below, that lowcapacity controller 28 provides precise capacity control for theseminimal capacity requirements. Minimum capacity requirements may occurwhen cooling a small office or zone during “off-hours” of “after-hours”,when there is low or no load available for the start up of the chillersystem, at seasonal fluctuations, and so forth. Moreover, it will becomereadily apparent in the ensuing discussion that low capacity controller28 may be further employed to take advantage of “free cooling,” byutilizing the cooling effect of outside air to provide direct cooling toa building.

Low capacity controller 28 includes a heater, in the form of a heatexchanger 96, and a secondary loop 98 interposed between chiller fluidsection 22 and heat exchanger 96. In a preferred embodiment, secondaryloop 98 includes an inlet conduit 100 in fluid communication with returnline 38 for conveying a portion of chiller fluid 34 from return line 38to heat exchanger 96.

Secondary loop 98 further includes an outlet conduit 102 having a firstbranch 104 selectively in fluid communication with supply line 36 and asecond branch 106 selectively in fluid communication with supply line 36of chiller fluid section 22. First means, in the form of a modulatingvalve 108, is positioned along first branch 104 of outlet conduit 102for selectively returning the portion of chiller fluid 34 to return line38. Modulating valve 108 enables a rate controlled flow of chiller fluid34 to return line 38 (discussed below). Second means, in the form of achiller side flow valve 110, is positioned along second branch 106 ofoutlet conduit 102 for selectively returning chiller fluid 34 to supplyline 36 (discussed below).

Condenser fluid loop 76 includes a secondary loop section 112 interposedbetween condenser fluid section 26 and heat exchanger 96. Morespecifically, secondary loop section 112 includes an inlet section 114in fluid communication with a condenser fluid supply line 116 ofcondenser fluid loop 76 and heat exchanger 96. Secondary loop section112 further includes an outlet section 118 in fluid communication withheat exchanger 96 and a condenser fluid return line 120 of condenserfluid loop 76. Means, in the form of a condenser side flow valve 122, ispositioned along outlet section 118 for selectively enabling a transferof condenser fluid 78 from condenser fluid loop 76 through heatexchanger 96 and back to condenser fluid loop 76 (discussed below).

In an exemplary embodiment, heat exchanger 96 is a plate and frame heatexchanger that provides a safe, sanitary, and efficient method oftransferring heat from one fluid medium to another as the two streamspass on opposing sides of intervening plates. In particular, heat istransferred between chiller fluid 34 and condenser fluid 78 in responseto a particular mode of operation in which chiller system 20 is beingoperated. This heat transfer between chiller fluid 34 and condenserfluid 78 will be discussed in detail in connection with the flowchart ofFIG. 2.

System controller 29 generally oversees, manages, and controls thevarious components of chiller system 20. System controller 29 mayencompass a wide variety of electrical devices (programmable or notprogrammable) having the ability to provide various output signals inresponse to various input signals. This communication is schematicallyrepresented by a bi-directional arrow 124. Examples of system controller29 include, but are not limited to, microcomputers, personal computers,dedicated electrical circuits having analog and/or digital components,programmable logic controllers, and various combinations thereof.

In addition, system controller 29 may be utilized to oversee and manageindividual controllers of various equipment groups. In such a situation,cooling tower 72 may be associated with an individual controller, eachof chillers 40 may be associated with an individual controller, andsignals may be forwarded between the individual controllers and systemcontroller 29. Such a controller is typically supplied from themanufacturer as a complete direct-digital-control (DDC) and monitoringpackage.

FIG. 2 shows a flowchart generally illustrating an operational process126 of chiller system 20 in accordance with the preferred embodiment. Inparticular, system controller 29 (FIG. 1) may control chiller system 20to provide the necessary capacity to satisfy a capacity demand, or load.Process 126 begins with a task 128.

At task 128, controller 29 determines the capacity demand. The capacitydemand is a measure of the amount of cooling required, i.e., the totalload, imposed upon chiller system 20 at a give instant. In a preferredembodiment, capacity demand is calculated as a function of the flow rateof chiller fluid 34 and the difference between the supply and returntemperatures of chiller fluid 34, as follows:DEMAND (tons)=FLOW RATE*(CHWST−CHWMRT)500/12,000where flow rate is detected at flow sensor 54 (FIG. 1), CHWST is thechilled water supply temperature measured at supply temperature sensor56, and CHWMRT is the chilled water mixed return temperature measured atreturn mixed temperature sensor 60. Although the above function may beutilized to determine the capacity demand, those skilled in the art willrecognize that alternative functions may be employed that provide ameasure of the capacity demand.

Next, a task 130 is performed. At task 130, controller 29 determines theambient conditions. Controller 26 can determine the ambient conditionsby receiving signals that include, for example, outside air temperature,outside air temperature dew point, and the like, from the appropriatesensors, as known to those skilled in the art.

In response to data collection tasks 128 and 130, a query task 132 isperformed. At query task 132, system controller 29 determines whetherload requirements and ambient conditions are such that chiller system 20may enter a chiller bypass mode, also known as an economizer mode. Asdiscussed above, under cool, dry outdoor conditions, the cooling effectof outside air may be utilized to provide direct cooling using coolingtower 72 (FIG. 1). When ambient conditions indicate such is the case,process 126 proceeds to a task 134. At task 134, chiller system 20(FIG. 1) is operated in a chiller bypass/economizer mode. Thereafter,process 126 loops back to tasks 128 and 130 to continue monitoringcapacity demand and ambient conditions. The chiller bypass/economizermode will be discussed in connection with a chiller system fluid flowschematic presented in FIG. 4.

However, when system controller 29 determines that load requirements andambient conditions are such that chiller system 20 should not enter achiller bypass mode, process flow proceeds to a query task 136. At querytask 136, system controller 29 determines whether the capacity demandcalculated at task 130, is less than a minimum allowable capacity. In apreferred embodiment, the capacity demand is compared with an internaltable of system controller 29 which is the cooling capacity of theinstalled chillers 40. The internal table preferably contains a minimumallowable capacity figure for chiller system 20. Since the individualcapacities for each of chillers 40 need not be the same, the minimumallowable capacity figure may be approximately eighteen percent of thetotal capacity of the lowest capacity one of chillers 40 in chillersystem 20.

When the capacity demand is not less than the minimum allowablecapacity, process flow proceeds to a task 138. At task 138, systemcontroller 29 provides the signaling necessary to operate chiller system20 in a nominal mode. For example, system controller 29 will signal adiscrete output to energize the required number of chillers 40. Toenable chillers 40, system controller 29 may generate the discreteoutputs to energize variable flow devices 48, as needed. In addition,system controller 29 may generate discrete outputs to close condenserside flow valve 122, chiller side flow valve 110, and modulating valve108 so that low capacity controller 28 is bypassed. Once one or morechillers 40 have been enabled, each chiller 40 may operate based ontheir stand alone product integrated controls. Thereafter, process 126loops back to tasks 128 and 130 to continue monitoring capacity demandand ambient conditions.

When the capacity demand is less than the minimum allowable capacity,signifying a very low desired capacity, tasks 140, 142, and 144 areperformed.

Referring to FIG. 3 in connection with tasks 140, 142, and 144, FIG. 3shows a block diagram of chiller system 20 operating in a low capacitymode 146 in accordance with the preferred embodiment. FIG. 3particularly illustrates fluid flow through low capacity controller 28of chiller system 20. In order to clearly demonstrate the fluid flow,all of the reference numerals set forth in FIG. 1 are not repeated inFIG. 3. Rather, only those reference numerals that further theunderstanding of fluid flow in low capacity mode 146 are shown in FIG.3. It should be understood, however, that the components of chillersystem 20 in FIG. 1 are equivalent to and present in chiller system 20illustrated in FIG. 3.

At task 140, system controller 29 provides the necessary signaling toroute condenser fluid 78 to heat exchanger 96 via inlet section 114 ofsecondary loop section 112. This is accomplished by opening condenserside flow valve 122 which enables a flow of condenser fluid 78 throughsecondary loop section 112.

At task 142, system controller 29 provides the appropriate signaling toroute a portion of chiller fluid 34, i.e., portion 34′, to heatexchanger 96 via inlet conduit 100 of secondary loop 98. Chiller sideflow valve 110 of secondary loop 98 is maintained in a closed position.Accordingly, portion 34′ of chiller fluid 34 is routed through heatexchanger 96 and returned to supply line 36 via first branch 104 ofoutlet conduit 102. Portion 34′ of chiller fluid 34 mixes with chillerfluid 34 supplied from chillers 40, as denoted in FIG. 3 by the numerals34+34′. Since chiller system 20 is operating at low capacity, much ofthis mixed chiller fluid 34+34′ is returned to return line 38 via bypassvalve 52 to maintain a minimum flow of mixed chiller fluid 34+34′ inchiller fluid loop 30.

At heat exchanger 96, heat is transferred from the warmer condenserfluid 78 to the cooler portion 34′ of chiller fluid 34. This causes thetemperature of portion 34′ of chiller fluid 34 to increase.Consequently, portion 34′ causes the chiller fluid in return line 38,i.e., mixed chiller fluid 34+34′, of chiller fluid loop 30 to be warmerthan what was originally measured at return mixed temperature sensor 60.This warmed chiller fluid 34+34′ establishes a “false capacity demand”from the perspective of chillers 40, so that chillers 40 will not enterinto an automatic chiller shutdown due to a freeze protection condition.

Low capacity mode 146 may be most clearly explained by example. Undernormal conditions, let it be assumed that the supply and returndifferential temperature of chiller fluid 34 is 16° F. when operating atminimum flow and full capacity. Under this condition with a designsupply temperature of 45° F., the temperature of returning chiller fluid34 should be approximately 61° F. If the minimum allowable capacity ofchillers 40 is eighteen percent, then the minimum allowed returntemperature is approximately 48° F. Anything below this temperature istoo cold for chiller 40. Because chiller 40 cannot download any further,chiller fluid 34 leaving chillers 40 drops below the set point of 48° F.Without the inclusion of the present invention, this “too cold”condition can cause the initiation of a freeze alarm, and cause chillers40 to shut down. By warming chiller fluid 34 in return line 38, thetemperature of chiller fluid 34 can be kept above this alarm condition,for example, above 52° F.

Modulating valve 108 is adjusted, i.e., modulated, to allow enough flowof portion 34′ of chiller fluid 34 into return line 38. This flow istied to the capacity demand formula presented above. That is, thetemperature of chiller fluid 34 is warmed so that the false capacitydemand detectable at chillers 40 is above the minimum allowablecapacity.

Following tasks 140, 142, and 144 operational process 126 loops back totasks 128 and 130 to continue monitoring capacity demand and ambientconditions. Chiller system 20 remains in low capacity mode 146 until thecapacity demand, as determined at task 128, becomes greater than theminimum allowable capacity, or unless chiller system 20 is able to enterthe chiller bypass/economizer mode as determined through ambientconditions at task 130.

FIG. 4 shows a block diagram of chiller system 20 operating in a chillerbypass/economizer mode 148 in response to task 134 (FIG. 2) of process126 (FIG. 2). FIG. 4 particularly illustrates fluid flow through lowcapacity controller 28 of chiller system 20 when in chillerbypass/economizer mode 148. In order to clearly demonstrate the fluidflow, all of the reference numerals set forth in FIG. 1 are not repeatedin FIG. 4. Rather, only those reference numerals that further theunderstanding of fluid flow in chiller bypass/economizer mode 148 areshown in FIG. 4. It should be understood, however, that the componentsof chiller system 20 in FIG. 1 are equivalent to and present in chillersystem 20 illustrated in FIG. 4.

In chiller bypass/economizer mode 148, system controller 29 provides thenecessary signaling to route condenser fluid 78 to heat exchanger 96 viainlet section 114 of secondary loop section 112. This is accomplished byopening condenser side flow valve 122 which enables a flow of condenserfluid 78 through secondary loop section 112. An “X” positioned over eachportion of condenser fluid loop 76 entering condensers 64 represents no-or minimal-flow conditions of condenser fluid 78 into condensers 64since chillers 40 may be deactivated in chiller bypass/economizer mode148. This may be accomplished by partially or entirely closing condenserinlet valves 94.

In addition, system controller 29 provides the appropriate signaling toroute chiller fluid 34 to heat exchanger 96 from return line 38 viainlet conduit 100 of secondary loop 98 and back to supply line 36 ofchiller fluid loop 30. This is accomplished by opening chiller side flowvalve 110 and maintaining modulating valve 108 in a closed position.Accordingly, chiller fluid 34 is routed through heat exchanger 96 andreturned to supply line 36 via second branch 106 of outlet conduit 102.An “X” positioned over supply line 36 directed away from chillers 40 andreturn line 38 directed toward chillers 40 represents no- orminimal-flow conditions of chiller fluid 34 into evaporators 68 sincechillers 40 may be deactivated in chiller bypass/economizer mode 148.

At heat exchanger 96, heat is transferred from the warmer chiller fluid34 to the cooler condenser fluid 78. This causes the temperature ofchiller fluid 34 to decrease. This cooled chiller fluid 34 issubsequently routed to air handler 42 to cool the space served by airhandler 42. Chiller bypass mode 148 is sometimes referred to as aneconomizer, “waterside”, or “free cooling” mode because cooling isachieved utilizing the cooling effect of outside air to provide directcooling instead of operating the energy intensive chillers 40.Accordingly, low capacity controller 28 achieves the secondary benefitof providing direct cooling, as well as enabling chillers 40 to operateunder very low load conditions by establishing a false capacity demand,as discussed above.

FIG. 5 shows a block diagram of a low capacity controller 150 forchiller system 20 in accordance with an alternative embodiment of thepresent invention. Low capacity controller 150 simply replaces lowcapacity controller 28 described in detail above. Accordingly, all ofthe reference numerals and components set forth in FIG. 1 are notrepeated in FIG. 5. Rather, only those reference numerals and componentsthat further the understanding of low capacity controller 150 are shownin FIG. 5.

Low capacity controller 150 includes heat exchanger 96, and a secondaryloop 152 interposed between chiller fluid section 22 (FIG. 1) and heatexchanger 96. In this alternative embodiment, secondary loop 152includes inlet conduit 100 in fluid communication with return line 38for conveying portion 34′ of chiller fluid 34 from return line 38 toheat exchanger 96.

Secondary loop 152 further includes an outlet conduit 154 having a firstbranch 156 selectively in fluid communication with return line 38 andsecond branch 106 selectively in fluid communication with supply line 36of chiller fluid section 22. Like low capacity controller 28 (FIG. 1),low capacity controller 150 also includes chiller side flow valve 110positioned along second branch 106 of outlet conduit 102 for selectivelyreturning chiller fluid 34 to supply line 36 when chiller system 20(FIG. 1) operates in the economizer mode (as discussed above).

A pump 158 is placed in series with modulating valve 108 along firstbranch 156 of outlet conduit 154 for selectively returning portion 34′of chiller fluid 34 directly to return line 38. Due to the higher flowrate and volume of chiller fluid 34 in return line 38, pump 158 isutilized to force the flow of portion 34′ of chiller toward return line38, while modulating valve 108 enables the rate controlled flow ofchiller fluid 34 to return line 38. Accordingly, portion 34′ of chillerfluid 34 and returning chiller fluid 34 are mixed in return line 38, asdenoted by reference numerals 34+34′, when chiller system 20 (FIG. 1)operates in low capacity mode 146 (FIG. 3).

As discussed above, heat is transferred from the warmer condenser fluid78 to the cooler portion 34′ of chiller fluid 34. This causes thetemperature of portion 34′ of chiller fluid 34 to increase.Consequently, portion 34′ causes the chiller fluid in return line 38,i.e., mixed chiller fluid 34+34′, of chiller fluid loop 30 to establishthe “false capacity demand” from the perspective of chillers 40, so thatchillers 40 will not enter into an automatic chiller shutdown due to afreeze protection condition.

Like low capacity controller 28 (FIG. 1), low capacity controller 150also includes chiller side flow valve 110 positioned along second branch106 of outlet conduit 154 for selectively returning chiller fluid 34 tosupply line 36 when chiller system 20 (FIG. 1) operates in theeconomizer mode (as discussed above in connection with FIG. 4).

FIG. 6 shows a block diagram of a low capacity controller 160 forchiller system 20 (FIG. 1) in accordance with another alternativeembodiment of the present invention. Low capacity controller 160 simplyreplaces low capacity controller 28 described in detail above.Accordingly, all of the reference numerals and components set forth inFIG. 1 are not repeated in FIG. 6. Rather, only those reference numeralsand components that further the understanding of low capacity controller160 are shown in FIG. 6.

Low capacity controller 160 includes heat exchanger 96, and a secondaryloop 162 interposed between chiller fluid section 22 (FIG. 1) and heatexchanger 96. In this alternative embodiment, secondary loop 162includes inlet conduit 100 in fluid communication with return line 38for conveying portion 34′ of chiller fluid 34 from return line 38 toheat exchanger 96.

Secondary loop 162 further includes an outlet conduit 164 having a firstbranch 166 selectively in fluid communication with return line 38 andsecond branch 106 selectively in fluid communication with supply line 36of chiller fluid section 22 (FIG. 1). A three-way valve 168 controls theflow of warmed portion 34′ of chiller fluid 34 into return line 38,thus, its subsequent mixing with returning chiller fluid 34 in returnline 38, while modulating valve 108 enables the rate controlled flow ofchiller fluid 34 to return line 38 (as discussed above. Accordingly,portion 34′ of chiller fluid 34 and returning chiller fluid 34 are mixedin return line 38, as denoted by reference numerals 34+34′.

Like low capacity controllers 28 (FIG. 1) and 150 (FIG. 5), low capacitycontroller 160 also includes chiller side flow valve 110 positionedalong second branch 106 of outlet conduit 164 for selectively returningchiller fluid 34 to supply line 36 when chiller system 20 (FIG. 1)operates in the economizer mode (as discussed above in connection withFIG. 4).

In summary, the present invention teaches of a method and low capacitycontroller for operating a chiller system. The low capacity controllerenables a chiller system to be operated below a minimum allowablecapacity for the chiller system by warming chiller fluid prior to itsreturn to the chillers. This establishes a false capacity demand at thechillers to prevent the chillers from going into an alarm and shut-downcondition. In addition, the low capacity controller achieves a secondarybenefit through its use in a chiller bypass/economizer mode to providedirect cooling through the utilization of the cooling effect of outsideair. The low capacity controller is readily and cost effectivelyincorporated into a chiller system through the use of a plate and frameheat exchanger, and an extension of the fluid loops to route the warmedchiller fluid back to the return line when in the low capacity mode andto route the cooled chiller fluid back to the supply line when in thechiller bypass/economizer mode.

Although the preferred embodiments of the invention have beenillustrated and described in detail, it will be readily apparent tothose skilled in the art that various modifications may be made thereinwithout departing from the spirit of the invention or from the scope ofthe appended claims. For example, the utilization of the heat exchangerin the chiller bypass/economizer mode may be optional. As such, thepresent invention may be adapted to include a heat exchanger thatutilizes a fluid other than the condenser fluid.

1. A chiller system for providing a fluid to a heat exchange unitcomprising: a chiller; a pump for forcing said fluid through saidchiller; a fluid loop having a supply line for conveying said fluid fromsaid chiller to said heat exchange unit, and a return line for returningsaid fluid from said heat exchange unit to said chiller; means fordetermining a capacity demand for said chiller; and when said capacitydemand is less than a minimum allowable capacity of said chiller, meansfor warming said fluid in said return line to establish a false capacitydemand, detectable at said chiller, that is at least said minimumallowable capacity.
 2. A chiller system as claimed in claim 1 whereinsaid determining means comprises: a first temperature sensor formeasuring a supply temperature of said fluid in said supply line; asecond temperature sensor for measuring a return temperature of saidfluid in said return line; means for determining a flow rate of saidfluid in said fluid loop; and means for calculating said capacity demandas a function of said flow rate and a difference between said returntemperature and said supply temperature.
 3. A chiller system as claimedin claim 1 wherein said warming means comprises: a heat exchanger; and asecondary loop in communication with said return line and in heatexchanging relation with said heat exchanger, a portion of said fluid insaid return line being routed through said secondary loop to warm saidfluid in said heat exchanger when said capacity demand is less than saidminimum allowable capacity, and said portion of said fluid beingreturned to said return loop via said secondary loop when said fluid iswarmed.
 4. A chiller system as claimed in claim 3 wherein: said chillersystem further comprises a bypass valve interposed between said supplyline and said return line; and said secondary loop comprises: a supplyconduit for transporting said portion of said fluid from said returnline toward said heat exchanger; a return conduit coupled with saidsupply line for conveying said portion of said fluid from said heatexchanger; and a modulating valve in fluid communication with saidreturn conduit for modulating a return of said portion of said fluid tosaid return line, said portion of said fluid being returned to saidreturn line via said bypass valve.
 5. A chiller system as claimed inclaim 3 wherein: said secondary loop comprises: a supply conduit fortransporting said portion of said fluid from said return line towardsaid heat exchanger; a return conduit coupled with said return line forreturning said portion of said fluid from said heat exchanger to saidreturn line; a modulating valve in fluid communication with said returnconduit for modulating a return of said portion of said fluid to saidreturn line; and said chiller system further comprises means forregulating a flow of said portion of said first fluid into said returnline.
 6. A chiller system as claimed in claim 5 wherein said regulatingmeans is a pump interposed along said return conduit.
 7. A chillersystem as claimed in claim 5 wherein said regulating means is athree-way valve positioned at a junction of said return conduit withsaid return line.
 8. A chiller system as claimed in claim 3 furthercomprising: a cooling tower; and a condenser fluid loop interposedbetween said chiller and said cooling tower, said second fluid loopbeing routed through said heat exchanger, and said condenser fluid loopconveying a second fluid through said heat exchanger when said capacitydemand is less than said minimum allowable capacity such that heat fromsaid second fluid is transferred to said portion of said fluid.
 9. Achiller system as claimed in claim 8 further comprising: means forselectively operating said system in a chiller bypass mode; means for,in said chiller bypass mode, deactivating said chiller; means for, insaid chiller bypass mode, transporting said portion of said fluidthrough said secondary loop for enabling heat transfer in said heatexchanger between said condenser fluid loop and said secondary loop tocool said portion of said fluid; and means for, in said chiller bypassmode mode, returning said portion of said fluid from said secondary loopto said supply line.
 10. A low capacity controller for a chiller system,said chiller system including a chiller and a first fluid loop, saidfirst fluid loop including a supply line for conveying a chiller fluidfrom said chiller and a return line for returning said chiller fluid tosaid chiller, said chiller being operable above a minimum allowablecapacity for said chiller, and said low capacity controller comprising:a heater; a secondary loop interposed between said first fluid loop andsaid heater; means for enabling a transfer of said chiller fluid throughsaid heater via said secondary loop when a capacity demand is less thansaid minimum allowable capacity, said chiller fluid being warmed at saidheater; and means for returning said chiller fluid from said heater tosaid return line via said secondary loop.
 11. A low capacity controlleras claimed in claim 10 wherein said chiller system further includes acooling tower and a condenser fluid loop interposed between said chillerand said cooling tower, said condenser fluid loop carrying a condenserfluid, and said low capacity controller further comprises means forenabling a transfer of said condenser fluid through said heater via saidcondenser fluid loop when said capacity demand is less than said minimumallowable capacity for said chiller.
 12. A low capacity controller asclaimed in claim 10 wherein said returning means comprises a modulatingvalve for modulating a return of said chiller fluid to said return line.13. A low capacity controller as claimed in claim 10 wherein saidchiller system includes a bypass valve interposed between said supplyline and said return line, and said secondary loop comprises: a supplyconduit for transporting said portion of said fluid from said returnline toward said heat exchanger; a return conduit coupled with saidsupply line for conveying said portion of said fluid from said heatexchanger, said portion of said fluid being returned to said return linevia said bypass valve.
 14. A low capacity controller as claimed in claim10 wherein said returning means adjusts a volume of said chiller fluidreturned to said return line in response to said capacity demand.